Harness parts for motor vehicles have conventionally been produced from a synthetic resin such as polyamide 6, polyamide 66, or polypropylene from the standpoints of strength, heat resistance, etc. However, harness parts made of such polyamide resins have been unsatisfactory in that thin-wall regions thereof (e.g., 0.2 to 0.5 mm) come into the absolute dry state due to the heat of the engine to have impaired toughness and that they have insufficient resistance to calcium chloride. Further, harness parts made of polypropylene have not always been satisfactory in log-term heat resistance and rigidity, although relatively satisfactory in toughness and calcium chloride resistance.
Since all the requirements of a harness part, i.e., toughness, heat resistance, and calcium chloride resistance, have thus been unable to be simultaneously met with any single material, harness parts made of different materials have been suitably used according to places of use.
With the recent improvement in the performances of motor vehicles, the number of parts employed in an engine room has increased and the resin moldings used therein have come to be exposed to atmospheres of higher temperatures than conventional ones. Hence, higher performances have come to be required of harness parts. If there is a synthetic molding resin which combines the properties of a polyamide and those of polypropylene, it is thought that these various performance requirements of a harness part can be met in a wider range.
It is however known that merely blending a polyamide resin with polypropylene only gives resin compositions which are not significantly improved in toughness and calcium chloride resistance. The reason for this is that since polyamide resins are inherently incompatible with polypropylene, intimate mixtures of the resins are not obtained. Several techniques have therefore been proposed so far for intimately blending a polyamide resin with polypropylene.
Proposed as expedients for improving the compatibility of a polyamide resin with polypropylene are a technique of incorporating an ionomer (as described in JP-B-43-6529), and a technique of using a modified polyolefin (as described in JP-B-45-30945). (The term "JP-B" as used herein means an "examined Japanese patent publication.") However, these techniques have failed to sufficiently improve calcium chloride resistance, although effective in improving compatibility.
A technique of chemically bonding the two resins by modifying each resin at terminals thereof has been proposed (as described in JP-B-64-56751). In this technique, a polyamide resin modified at terminal amino groups is melt-kneaded with a polypropylene modified with an unsaturated carboxylic acid using a single-or twin-screw extruder. However, there is a problem that since terminal groups of either resin react with terminal groups of the other during kneading to increase the melt viscosity, the composition has impaired moldability and molding of thin-wall products is adversely affected because unevenness of wall thickness is apt to result.